This invention relates to a control for a solid state adjustable speed induction machine drive and more particularly to a control for achieving fast transient response.
Traditionally, induction motors have been controlled by scalar methods where all the control and feedback signals are processed in scalar (d.c.) form. Since an induction motor is basically a coupled system where a variation of either the voltage or frequency applied to the motor stator affects both the motor torque and rotor flux, scalar controls using feedback give inherently sluggish response. A commanded increase in the torque results in an initial temporary dip in the rotor flux due to inherent coupling which in turn decreases the motor torque. The feedback signal will gradually respond to restore the rotor flux and the commanded torque will be achieved.
Another type of induction motor control uses vector control methods. The a.c. machine is controlled as a d.c. machine by decoupling the flux and torque components of the stator currents. There are principally two different vector control methods. The first method is the closed loop or direct vector control method which achieves decoupling by using feedback of the rotor flux vectors and is used typically when operating above 10% of base speed where the flux vectors can be estimated or measured with reasonable accuracy. The second method is the open loop or indirect vector control method which depends on feedback of rotating electrical vectors which are synthesized by the addition of a slip angle to the rotor mechanical position vector. The open loop method can be used over the entire speed range including zero speed. The vector control methods while effective during steady state operation, strictly speaking, only achieve quasi decoupling during current transient conditions. The phasor diagrams used to explain the vector control operation are not strictly valid with respect to transient conditions when the machine response is governed by the direct-quadrature (d-q) axis equations. The quasi decoupling achieved is more decoupling than would be achieved without the control but less than full decoupling.
In applications where fast response is needed such as in servo applications and in motors used in driving rolling mills, d.c. machines are mainly used at present.
It is an object of the present invention to provide an induction motor control that achieves fast response at various operating points using a simplified control.
It is a further object of the present invention to achieve decoupling between the commanded frequency and rotor flux in a scalar control during static and dynamic conditions at all operating points.
It is a still further object of the present invention to provide a scalar decoupled control having fast transient response and four-quadrant operating capability.